The U.S. Food and Drug Administration said that the agency is close to identifying the unknown contaminant in some lots of the recalled blood-thinner heparin sold by Baxter International Inc. The contaminant found in the drug acts like heparin, the FDA said. Therefore, it would not show up on traditional tests.

Samples that tested positive contained 5 percent to 20 percent of the contaminant. The agency said it could not yet identify how the substance got into Baxter’s product and whether its introduction was deliberate. Heparin is derived from a substance in the lining of pig intestines, and the contamination could have occurred at a variety of stages in its production.

"We are very close," FDA Deputy Commissioner Janet Woodcock said on Friday. "When we complete that, [it] will help our understanding of how it got in."

The other major U.S. heparin supplier, APP Pharmaceuticals Inc, ramped up production to avoid a drug shortage after Baxter recalled most of its heparin products last week. Tests on APP's heparin have not turned up any contamination.

"There are three key questions that we are trying to answer," said Norbert G. Riedel, Baxter's chief scientist, quoted by LA Times. "What exactly is the contaminant, where in the supply chain was it introduced, and can we prove that this contaminant is in fact causing the reactions we've seen?"

Last month, Baxter recalled the bulk of its heparin products after receiving hundreds of reports of allergic reactions possibly linked to heparin, as well as reports of four deaths. Two weeks ago, the company recalled all remaining heparin products.